Manufacture of thiourea



Patented July 18, 1944 MANUFACTURE OF 'I'BIOUBEA Robert 11. Cooper,Nltro, W. Va., asallnor to Monsanto Chemical Company, St. Louis, Mo., a

corporation of Delaware No Drawing. Application September 22, 1941,Serial No. 411,883

2 Claims.

The present invention relates to the production of thiourea.

It is known that thiourea can be produced by the interaction of acyanamide and an inorganic sulfide, hydrogen sulfide, calcium sulfide,barium sulfide and ammonium sulfide all having been proposed forreaction either with free cyanamide or salts thereof as for examplealkaline earth metal salts, to produce thiourea.

Unfortunately the cyanamides are easily decomposed and readily undergorearrangement to dicyandiamid and other products. While it has beenproposed to carry out the thiourea synthesis at low temperatures inorder to avoid the formatlon of undesirable by-products, the timerequired for reaction is then unduly prolonged. It has been found moreeconomical to, maintain the inorganic sulfide in large excess over theamount required to combine with all of the cyanamide present in thereaction zone since i under these conditions the reaction can beconducted at elevated temperature without the formation of undesirableby-products. In other words the speed of the desired reaction has beenfound to be increased so enormously that the side reactions arenegligible. Best results have been obtained by conducting the reactionat 80-95 C. Obviously the unreacted portion of the inorganic sulfidemust be recovered for future use or accepted as a complete loss. Ofcourse even the time and expense of recovery represent an appreciableitem.

In accordance with the present invention it has been discovered thatsubstantially stoichiometric amounts of a cyanamide and an inorganicsulfide can be converted into thioureaby maintaining a large excess ofinorganic sulfide throughout the greater part of the reaction and thenfinishing off the reaction by adding a cyanamide in an amount such as torender the total only slightly below the stoichiometric equivalent ofthe inorganic sulfide. The reaction is almost instantaneous at elevatedtemperatures and at 100 C. or below the transfer of thiourea intothiocyanate is negligible.

From the standpoint of cheapness of reactants the interaction ofhydrogen sulfide and crude calcium cyanamide is particularly attractive.The reaction follows with reasonable conformity the equation:

or in the dry,

aqueous medium the hydrogen sulfide being in stoichiometrlc excessthroughout the entire reaction and in large excess throughout thegreater part of the reaction. However, the addition of cyanamide iscontinued after the addition 0! the hydrogen sulfide until the totalamount is only slightly less than the stoichiometrlc equivalent of thehydrogen sulfide. While at every stage of the reaction there is presenthydrogen sulfide in amount greater than required to react with all ofthe cya'namide present in the reaction zone, the total quantitiesreacted are in almost equal molecular proportions. In two runs identicalin every respect except that in one the hydrogen sulfide was kept atfrom 50-60% excess of that theoretically required whereas the other wasconducted as outlined above, the yield and purity of thiourea wereidentical.

Where fertilizer grade calcium cyanamide is used considerable quantitiesof carbon and, as

appears from Equation 1, calcium hydroxide are produced as well as smallamounts of other ingredients. The lime being gelatinous and slimyrenders filtration difficult and if the batch is neutralized and all 0!the calcium precipitated together as calcium sulfate or other insolublesalt the bulk of precipitate is greatly increased and a seriousfiltering problem still remains. However it has ben further found inaccordance with this invention that the reaction cycle is speeded up andthe quantity of thiourea which can be produced in a given plant isincreased by adding only about half of the total cyanamide to bereacted, filtering the charge without neutralizing, thenadding the otherhalf of the cyanam'de and, at agduced rate, hydrogen sulfide. At thehalf way stage the hydrogen sulfide is in large excess probably asCa(SH) a. In this connection it will be noted that Equation 1 falls toshow the formation of Ca(SH)s, CaS and Ca(0H) SH, all 01' which areundoubtedly present. The bulk of the lime, carbon and other solids isnot so great as to impede filtering so that after a short filteringoperation a clear filtrate is obtained ready for the second half of thereaction. It is important to note that the excess of hydrogen sulfide inwhatever form it may exist is available for reaction with additionalcyanamide. Accordingly, the flow of hydrogen sulfide can be stoppedaltogether while the final portions of cyanamlde are added. When thetotal hydrogen sulfide is only slightly in excess of the cyanamide theremainder of the calcium is precipitated as sulfate or other insolublesalt and thiourea recovered from the filtrate in known manner.

Example About 1250 lbs. of water are charged into an iron reactor ofsuitable capacity. It is preferred to 2 assess? mly'c ednse' hydrogensulfide in this water beiaer and when perfectly clear is pumped into I.tore adding an: cyanamide but i! desired the concentrator. Thecakeisweshed with hot water addition, t tsmanba l M a t ashinss alsopumped to the conceno ti lige gelsting ystals oi thiourea are obtainedalter may be a dd'd to ewhter andcnodzh 5 'ienlov'ing a portion of thewater, cooling and sulfide added to convert the lime to calcium by;filtering. Before the separation of the thiourea drosulfide. The chargeis heated to 85-90 C. and "however it is advisable to run the hotconcentrated gaseous hydrogen sulfide run in atwiflimt m i li hltiwthrough a filter to remove additional rate. Simultaneously theregl iihcrude ger sg nall amounts of impurities which frequently tilizer gradecalcium cyanfi 'g iz f al-ldu iiljepa' to as the solutionis'concentratedt 'Ihiopurity is gradually run oi by 'tnesns it their:high purity is obtained directly from the conveyor. In this manner 400lbs. of calcium concentrated solution. The yield is excellent. cyanamideare brought into IGGflOlifiThflFhYdi'Or-Li iii-awhile the aboveindicates a preferred ionn or gen sulfide is measured by means oi aEthe'improved process it is to be understood that how meter and the rateof fiow is so adiusteg tltat numerous changes can be made as for examplein an excess is always present. After a portionj the kind or theingredients used, the mode of addisoneeiourthfibtethegcaiciunmcyanamidehsszbeen $1011 or association or the ingredients and in theaddeditheeriiteintiadditiohzofdaydrogenrsulildenis temperature andpressure adopted for eflectins 2' decreased:somejvhatiaimtmaftendflilzlhsnot;eyanreaction. Such deviations can bemade by those iamide'ihaveaheensaddedi thesexcess oi'tihydmzen 2oskilled in the art without departing from the sslilfidenhouldstlllibetfromghwtofideizzabovethat principles outlined above and arewithin the scope dheoreticallysrequiredstu conuertializoizthe-icyanoi.the invention as set forth in the claimsatllfiildEZMLflflQliIEBiiEQUl-HOWIiUbOYE)?t g tached hereto as part ofthe present specification. ori;=-The;-chargeeis;themdedinsa.continuous-stream What is claimed is: ."intoiii'l CBnmIgEEq-ndetheefluielfilflaketmashefi'lwith In e process of making t urea by eatiwatemr Thezflitl'atetand washings; are mumped ins c l i m cyimflmide wh hydrogen sulfide the hack into) arreactor; the-:.-ten perature.: againadsteps or initiating the reaction by adding calcium&Jlls,ted.to::i!5,-90 :Ga and they additiomof hydrogen cyanamide tohydrogen sulfide in an amount sub-.rsulfideandcalciumhyanamideeontinued;the rate stantially gre ter h nthe stoichiometric equivarotg-addition-ot theformenibeinszreduced-stili-iurlent of the calcium cyanamide added to thereac- .ther 1and;sstopped:-=altogether ,duringi the: addition tion zone,maintaining throughout the reaction,oi'ethei;llasta:portions;of;theecyanamidee By the and any-subsequentadditions of the reactants. time;;.arn:itherlA90tibssoicaiciunrcyanamide have the hydrogen sulfide instoichiometrie excess of been added the total hydrogen sulfidershould bethe total calcium cyanamide present in the reionly;i..2.5e.;to;5.q%-aboveithe theoreticala-wequireaction zone at any time and finallyconverting "meat-n.- Thus based Qmthe overall: reaction an exnearly allof the hydro en sulfide to thlourea by aces-tut About ,60 %ir,is,built:umandanaintained adding further calcium cyanamide. .dllmszaboutthedlrst quartenofgthe reaction. By 2. In the process of makinthiourea by react- -the tirnethe -,halt';vva ;maric is reached the exingcalcium cyanamide with hydrogen sulfide the .cess is about 39%;,and-1-athathreequarter mark 40 steps of initiating the reaction by addingcalcium ;a hout;d' t%;; :2" cyanamide to a hot aqueous medium containinn a continuous hyrogen sulfide in an amount substantially greater t ercake washed than the stoichiometric equivalent of the calauu nta -1ceutrilleezthemfl yith states and; theyfiltrateand' washings pumped ciumcyanamide added to the reaction zone, main- .intgg-a;glassdiped;neutraiialnggtank. 68% sultaming throughout the reaction andany subse ed- -sl o\yly-' to the neutralizer until quent additions ofthe reagents, the hydrogen sulcharge ist; aintlya d; (pH 6.6). Thetemilde in stoichiometric excess of the total calcium PQIBIIQIQQQQL-UQQG. andgafterashort period cyanamide present in the reaction zone at anygig; eating andstirring the sulfate cake is filtered time and finallyconverting nearly all of the hyoil-lmffiimbl rhlixmeans of,-acentrifugal wringer 5g" drogen sulfide to thiourea by adding furthercalwith; castgaluminum basket. If necessary the cium cyanamide. filtrateis recirculated back through the neutral- ROBERT H. COOPER.

